It is desirable in optical wavelength-division multiplexing networks to have inexpensive light modulators that have high contrast and wide optical bandwidths. In certain cases, such as audio and video transmission, these modulators need only operate at frequencies up to several megahertz.
A modulation device particularly well suited for the above application is a surface normal micromechanical modulator. This device may be described as having a variable air gap defined by two layers of material. Typically, surface normal light modulators operate by changing the amount of light reflected in the surface normal direction, i.e., the direction normal to the substrate surface. This may be achieved by varying the variable air gap, which alters the optical properties of the device.
Compared with other modulation means, such as a laser, micromechanical modulators are limited in terms of modulation frequency. However, the micromechanical modulators are less expensive to implement and are readily fabricated on silicon substrates facilitating integration with silicon based electronics. Further, unlike the typical semiconductor laser, micromechanical modulators operate in a surface normal manner. This is an attractive feature since a device which operates in this manner requires less wafer space than a device, such as a typical semiconductor laser, in which the operating cavity is formed in the plane of the wafer. Many thousands of surface normal modulators may be formed on a single wafer, minimizing cost. Thus, where the operating frequency is limited, the micromechanical modulator may be the modulation device of choice. One such micromechanical modulator has been described by Aratani et al. in "Process and Design Considerations for Surface Micromachined Beams for a Tuneable Interferometer Array in Silicon," Proc. IEEE Microelectromech Workshop, Ft. Laud., Fla., Feb. 7-10, 1993 at 230-35. This article, and all other articles referenced in this specification are herein incorporated by reference in their entirety. Aratani's modulator is described as having a diaphragm mirror consisting of a polysilicon/silicon nitride multilayer supported by thin beams over a substrate, also partially mirrored by a polysilicon/silicon oxide multilayer. As a voltage is applied between the membrane and the substrate, the membrane is pulled toward the substrate. The device is said to behave as a Fabry-Perot interferometer wherein, given two mirrors having equal reflectivity, the reflectivity of the device approaches zero at the resonant wavelength of the cavity. As the membrane moves, altering the cavity, the reflectivity of the device rises. The change in reflectivity modulates the optical signal. While a large change in reflectivity is said to be achieved, the optical bandwidth of the optical resonator based modulator is limited. The contrast ratio of such a device falls off sharply as the wavelength of the incident light varies from the resonant wavelength of the device.
U.S. Pat. No. 5,500,761 discloses a mechanical modulator that is formed on a semiconductor wafer. The modulator includes a membrane and a substrate, spaced to form an air gap. The membrane consists of one or more layers, and is suspended over the substrate by support arms. Bias is applied to the membrane and the substrate to create an electrostatic force to move the membrane towards the substrate. The layers of the membrane are characterized in that there is a relationship between the refractive indices of the layers and the refractive index of the substrate. Each layer of the membrane has a thickness which is approximately equivalent to one-quarter of the wavelength of an optical signal to be modulated. In operation, the air gap, in the unbiased state, is a multiple of one-quarter of a wavelength of the optical signal. Where the air gap is an odd multiple of one-quarter wavelength, the membrane and air gap function as a high reflectivity coating. Where the air gap is an even multiple of one-quarter wavelength, the membrane and air gap function as an anti-reflection coating. Under the action of bias, the membrane moves through one-quarter of a wavelength to an anti-reflection state or a maximum reflection state depending upon the state of the unbiased membrane. In the second arrangement, the membrane does not contact the substrate. One advantage of this modulator over that disclosed by Aratani et al. is that it offers a broader resonance due to its large finesse. However, whereas the Aratani et al. modulator provides high contrast (i.e., the ratio of light reflected in its reflective state to its anti-reflective state), the modulator disclosed by Goossen et al. achieves a reflectivity of no more than 72% and thus its insertion loss is undesirably high for many applications.
It would therefore be desirable to provide a mechanical modulator that offers a broad resonance with low insertion loss.